Modern wireless systems often use multiple-input and multiple-output (MIMO) transmission techniques to achieve higher data rates. MIMO transmission generally refers to the utilization of multiple antennas at the transmitter and/or receiver, and includes a variety of diverse techniques including spatial multiplexing, diversity coding, pre-coding, etc. In particular, using spatial multiplexing by sending multiple spatial streams for a single user is quite useful in high-interference networks because it splits a relatively high data-rate signal into multiple lower data-rate streams, which helps lower the bit-error-rate (BER) of the communication channel. Notably, each of the lower data-rate streams is transmitted by a different transmission antenna port in a common frequency channel, with each individual stream being recognized upon reception by its unique spatial signature (e.g., transmit and/or receive weights). Hence, spatial multiplexing essentially enables the transmitter to communicate parallel streams (e.g., spatial multiplexed layers) of information over the same frequency band.
The maximum number of spatial multiplexing streams/layers (Ns) is limited by the lesser of the number of antennas at the transmitter (Nt) or the number of antennas at the receiver (Nr), e.g., Ns=min(Nt, Nr). Generally speaking, additional layers of spatial multiplexing increase spectral efficiency in the channel, thereby allowing for greater throughput, simultaneously supporting more transmitting users, etc. For instance, a network supporting eight layers of spatial multiplexing (e.g., Ns=8) may generally outperform a network supporting only four layers of spatial multiplexing (e.g., Ns=4). As such, modern communications standards are trending towards the utilization of more and more layers of spatial multiplexing (i.e., the use of more antenna ports). For instance, 3rd Generation Partnership Project (3GPP) long term evolution (LTE) release 10 (rel-10) specifies supporting up to eight layers of spatial multiplexing (e.g., 8 antennas at the transmitter and receiver).
To utilize spatial multiplexing effectively, the transmitter must generally have some knowledge of the communications channel, which (in the context of LTE) is generally referred to as channel state information (CSI). Specifically, CSI is obtained when a user equipment (UE) or relay node (RN) performs channel estimation on a reference signal that is propagated through the downlink channel, e.g., the physical downlink shared channel (PDSCH) or physical downlink control channel (PDCCH). The CSI may then be feedback to the base station directly (e.g., via an uplink control channel), or indirectly (e.g., by sending an indicator related to the CSI using channel reciprocity for a Time Division Duplexing (TDD)). In 3GPP LTE rel-10, the reference signal may be a dedicated/de-modulation reference signal (DM-RS) capable of supporting up to eight layers.